See EWG ( like SAE ) rules 76/115/EWG from 18th december 1975 under 4.5.1
thread must be 11,11 mm ( 7/16 ) 20 UNF 2B for all seat belts.
The head is free, for example Mercedes use most 17mm head, sometimes Torx. Sometimes with a shaft. Most people think it's a M11 x 1 or a M11 x 1.125, but that is wrong, it is 7/16.
